Southeastern Asia: Thailand, Cambodia, Laos, and Vietnam

The Central Indochina Dry Forests [IM0202] ecoregion covers most of central Indochina and harbors an outstanding assemblage of threatened large vertebrates that characterize the mammal fauna of the Indo-Pacific region. Just half a century ago large populations of megaherbivores such as Asian elephants (Elephas maximus), banteng (Bos javanicus), kouprey (Bos sauveli), gaur (Bos gaurus), wild water buffalo (Bubalus arnee), and Eld's deer (Cervus eldii) roamed and grazed in these dry woodlands (Wharton 1957). Where human densities were still low, the landscapes were dominated by large herds of wildlife reminiscent of the savannas of east Africa. Large carnivores such as tigers (Panthera tigris), clouded leopards (Felis nebulosa), leopards (Panthera pardus), and packs of wild dogs (Cuon alpinus) hunted these herbivores. Unfortunately, throughout the ensuing years habitat loss and hunting for trade have exacted a devastating toll on these species. Some species have even become extinct. The two rhinoceros species, the Javan (Rhinoceros sondaicus) and the Sumatran (Dicerorhinus sumatrensis) are now extinct in this ecoregion, as is Schomburgk's deer (Cervus duvaucelli schomburgki). The kouprey probably is globally extinct, although intermittent reports from remote areas of northern and eastern Cambodia keep hopes alive. Among the other species, the tiger, Asian elephant, Eld's deer, banteng, and gaur are endangered.

Scientific Code

(IM0202)

Ecoregion Category

Indo-Malayan

Size

123,600 square miles

Status

Vulnerable

Habitats

Description Location and General DescriptionThe Central Indochina Dry Forests [IM0202] ecoregion covers more area in mainland southeast Asia than any other forest type. It extends widely in Thailand, from dry lower slopes in northern Thailand and the foothills of the Tenasserim Range to uplands around the Chao Phraya Basin and then across the Khorat Plateau. In remains extensive in coverage along the broad valley of the Mekong and its tributaries in central and southern Laos and has a widespread distribution in the arid plains of northern, eastern, and south-central Cambodia. A small area of the Central Indochina Dry Forests [IM0202] ecoregion reaches into Vietnam within the upper watersheds of the Xe San and Xrepoc rivers. Over this range the ecoregion characteristically occurs in areas with 1,000-1,500 mm rainfall and five to seven months of drought. Potential evapotranspiration may exceed rainfall for up to nine months per year.

Deciduous dipterocarp forest, the name commonly used for the characteristic forest association of the Central Indochina Dry Forests [IM0202], forms an open forest or woodland community dominated by deciduous trees. This forest formation has been called idaing in Burma and forêt claire in Laos and Cambodia. Community structure may range from a nearly closed canopy forest of low trees 5-8 m in height, to more typical woodland structure with 50 to 80 percent canopy cover and an open understory dominated by grasses. Occasional emergent trees may reach to heights of 10-12 m. Along gradients of increasing environmental stress, whether from natural drought or human intervention, dry deciduous dipterocarp communities become increasingly open in structure and lower in stature, grading eventually into savanna woodlands with decreasing woody cover.

Deciduous species of Dipterocarpaceae form the dominant element of deciduous dipterocarp forests. Only six species of the approximately 550 dipterocarps in the world are deciduous, and all of these occur in this formation. Four of these, Shorea siamensis, S. obtusa, Dipterocarpus obtusifolius, and D. tuberculatus, generally form the dominant biomass and cover. The community often is moderately rich in other small trees, particularly legumes such as Pterocarpus macrocarpus, Sindora siamensis, and Xylia xylocarpa. Terminalia alata and Pinus merkusii may be a co-dominant species. Cycads are common in the grassy understory. Epiphytic vascular plants and lichens are few in number and low in diversity in lowland habitats of this ecoregion but increase in abundance with higher elevations and more humid conditions.

Ground fires burning through the herbaceous understory of deciduous dipterocarp forests are a regular aspect of the environment, so this association is sometimes called a fire climax community. The question, therefore, is how much of this community has been formed by a history of human activities that have greatly increased the frequency of such fires. Some researchers believe that a significant portion of the modern coverage of this habitat represents a type conversion of what was once semi-evergreen forest into deciduous dipterocarp forest under the influence of repeated burning. Other researchers believe that such conversions have been limited. Most fires occur between December and early March, when forest conditions are driest. Dominant tree species in this formation exhibit adaptations to fire in the form of thick, corky bark to protect cambium tissues and root crowns, which readily resprout.

Biodiversity FeaturesThe ecoregion's 167 mammal species include an impressive assemblage of threatened large vertebrates such as the critically endangered kouprey and Eld's deer, the endangered tiger, Asian elephant, gaur, banteng, wild water buffalo, serow, and other species such as the pileated gibbon, two leaf monkey species (Semnopithecus cristatus and S. phayrei), wild dog (dhole), Malayan sun bear, clouded leopard, and common leopard. It also harbors two endemic species of Vespertilionidae bats (Myotis altarium and Pipistrellus pulveratus) (table 1).

An asterisk signifies that the species' range is limited to this ecoregion.

The remaining large habitat blocks overlap with six high-priority (Level I) and two Level II TCUs (Dinerstein et al. 1997). But the long-term conservation success of tigers and the ecological integrity in these conservation landscapes will be compromised if the habitat becomes fragmented.

Many of the 500 or so bird species include several that are of conservation importance for their role as focal species for conservation management and for their threatened status. The latter include the critically endangered white-eyed river-martin (Pseudochelidon sirintarae), the globally threatened Bengal florican (Eupodotis bengalensis), and the endangered greater adjutant (Leptoptilos dubius) and white-shouldered ibis (Pseudibis davisoni).

An asterisk signifies that the species' range is limited to this ecoregion.

The reptile and amphibian faunas are not well known. But the limited information suggests that the critically endangered turtle Pelochelys cantorii, the geckos Gehyra lacerata and Gekko petricolus, the agamid lizard Ptyctolaemus phuwuanensis, the two skinks Isopachys borealis and Lygosoma koratense, the earth snake Typhlops khoratensis, and the colubrid snake Oligodon hamptoni are ecoregional endemics. According to local reports, critically endangered Siamese crocodiles may persist at some isolated permanent lakes.

Current StatusAlthough large blocks of forests remain in northeastern Cambodia, most of the natural habitat has been extensively cleared in Thailand, Laos, and Vietnam. There are twenty-one protected areas that cover more than 15,000 km2 (6 percent) of the ecoregion. These include four (Phu Kao-Phu Phan Kham, Xe Piane, Phnom Prich, and Yok Don) that exceed 1,000 km2 and two (Kulen Promtep and Lomphat) that exceed 2,000 km2 (table 3). Many of these large protected areas, in Laos and Cambodia, were established recently and represent parts of the ecoregion where habitat still remains. But large areas of the ecoregion now in northeastern Thailand are deforested and lack protected areas.

Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.

Types and Severity of ThreatsMost of the ecoregion lies in densely populated areas, where the natural habitat has long been converted to agriculture and settlements. Some fires are natural, but many are set to clear land for shifting agriculture and to promote new growth of grasses for livestock, to make traveling easier, and to attract wildlife, making hunting easier. In Thailand the Hmong and Yao hill tribes, who have traditionally cultivated upland rice and opium poppy, have been moving progressively further south and into lower-elevation dry forests and have begun clearing large areas of habitat (IUCN 1991). Areas of this ecoregion that are sparsely populated may be threatened by paddy rice development (encroachment) and the associated human population increase. Parts of Cambodia, Laos, and Thailand are threatened by dam development. In Cambodia, large reservoirs are proposed for the lower Se San and Srepok rivers. Some Thailand protected areas suffer from tourism. The land is being degraded, and rubbish is commonly discarded. In Laos, protected areas suffer from shifting cultivation, excessive NTFP harvesting, wildlife poaching (for domestic use and local and international trade), and illegal logging.

Justification of Ecoregion DelineationIn his analysis of the conservation units of the Indo-Pacific realm, MacKinnon (1997) included the dry deciduous forests in the single Indochina biounit (10), along with other vegetation types. In keeping with our rules for defining ecoregions, we extracted the dry deciduous forests and placed them in the Central Indochina Dry Forests [IM0202].

ReferencesReferences for this ecoregion are currently consolidated in one document for the entire Indo-Pacific realm.Indo-Pacific Reference List